Apparatus for forming bonded granular articles



March 7, 1939- R. L. MELTON ET AL APPARATUS FOR FORMING BONDED GRANULAR ARTICLES Fild Oct. 15. 1955 INVENTOR. ROMIE L. MELTON HENRY F? KIRCHNER BY RAYMOND 5. EEMNEFE ATTQ Patented Mar. 7, 1939 UNITED STATES PATENT OFFICE APPARATUS FOR FORMING BONDED GRANULAR ARTICLES of Delaware Application October 15, 1935,"Serial 'No. 45,142

" Claims.

This invention relates to granular bodies in which the individual particles are orientedina predetermined. direction, and more particularly to abrasive articles containing oriented abrasive 5 grains. The invention also relates to the orientationof particlesinan uncured mix eitherprior to or during consolidation. An important object of the invention is the attainment of improved cutting or grinding, characteristics in a solid abrasive article by the orientation of the abrasiveggrains so as toutilize themaximuin abradingpropcrties of such crystalline orgranular particles.

This application is a continuation in part of'our copending application, Serial No. 635,232, filed September 28, 1932.

In the manufacture of abrasive articles, the grains used vary considerably in shape, but may be broadly classified into three general classes:

1. Equiaxed particles in which all three dimensions of the individual grain are approximately equal.

,2. Particles in which two dimensions are appreciably greater than the third, an extreme ex- .ample of a grain of this type being a thin crystal plate, which is almost two-dimensional.

3. Elongated particles in which one axis appreciably exceeds the other tw0,-an extreme ex- ;ample of this type of grain being; a sharp needle or splinter.

v30 Commercial abrasive grain as ordinarily manu- .factured is a mixture of all three classes of particles and with somematerialsthere is a marked tendency for at least one axis of the grain to be greater than the other two axes. As an example, the plate-like structure of silicon carbide is well known. In the usual abrasive wheel, the irregular shaped or elongated particles are positioned entirely at random and exceptfor a few occasional particles the sharpupoints or cutting edges v40 characteristic of certain portions of the particles are not utilized for cutting or grinding.

In accordance with our invention, we orient the individual grains within the abrasive article so that their greatest dimension lies in 37661133111 predetermined direction. We have ,found that by preferentially orienting the abrasive particles, the cutting action which is obtained during grinding or abrading can be greatly increased. The exact nature or direction of orientation employed will depend upon the specific grinding 55 tion of elongated crystals is very desirable, and

produces a grinding wheel in which the face is composed of a;s eries ,of sharp projecting teeth. -With'such,a eel the gouging orcutting effect .ismuch greater than .withthe ordinary abrasive wheclhand t e frictional W ar c d .bythc exposure of fiat surfaces ,of'thetabrasive grainsto the Work is minimized orpractically eliminated. Orientation in other directions, also is. of advantage in certain specific instances, and in accord- ;ance with the processes herein described, the direction of crientation can be predetermined so ,as toproducethe specificeffecthdesired. For example, we may orient the elongated grains .or splinter y type somewhat at ,an angle so that the cuttingaction is similartothat ofa sawtooth. .Ina wheel ordisc in which the side surface is to b use t sra n a b ori nt d at ri ht an les v,to the surface of the wheelinstead of orienting .the ,particles in a radial direction, so that the ,sharp portions of the grain willproject from the working surface. Other directions of v orientation may. alsobe desirablefor certain;particular uses.

By specially selecting the grain used in making the abrasive article, it is possible to greatlyaccentuate the properties resulting from orienta- ,tion. Grains of-a pronounced flaky or splintery character ordinarily are not considered as desirable for abrasive purposes, but we have found that these grains can be oriented so as to present very .sharp cutting edges at theexposed surface of the article. Our invention thus-turns to actual advantage certain properties which heretofore have been considered as undesirable in abrasive grains, and makes possible the use of certain types of grains which in the past have been subject to veryelaborate treatment in order to effect their removal from. the material used in making high grade .abrasive articles.

Inmaking an article in which the individual particles are oriented, various methods of fabrication can beused. The grains can. be oriented by electrostatiameansprior to the final consolidating operation, vor when certain fabrication ;methods suchas jolting-are used, orientation and consolidation can be made to take place simul- .taneously. -If a mixcontaining plastic bond is consolidated by jolting, the grains in the consolidated product can be oriented in parallel planes "without anypreliminary orientation in the unconsolidated mix. If desired, such a processcan be combined with preliminary orientationprior to consolidation, and in certain instances two types ;of orientation, as for example, radial orientation and orientation in whichthe fiat surfaces of theparticles lie in, parallel planes, .can be v represents an optimum condition with a grain of this character.

A third type of orientation, in which the axes of the particles are parallel, but in which the fiat surfaces of the particles do not necessarily lie in'parallel planes, can also be produced either by orientation of the particles of the mix within the mold, or by electrostatic projection of the 7 particles during the formation 'of the article.

Several methods of carrying out our invention are shown in the accompanying drawing.

In the drawing:

Figure 1 shows diagrammatically a section of a mold which can be used for the electrostatic orientation of the particles within an abrasive wheel so as to produce radial orientation, and a screen for filling said mold; and

Figure 2 shows a method of orienting the axes of elongated particles so that they are substantially parallel.

In making an'article in which the particles are to be oriented, the usual grains such as are employed for the manufacture of abrasives can be used. It is desirable however, in many cases, to specially select or prepare the grain so that the greatest possible advantage can be derived from orientation. There are various methods known in the art for obtaining a grain in which one or two dimensionsconsiderably exceed the third dimension, or of separating such grain from the more nearly equiaxed particles. When grain of this type is preselected, it is possible to obtain an abrasive article possessing unusual characteristics.

Various crushing and screening methods can also be used for obtaining elongated or plate-like grains. For example, roll crushing will produce a greater proportion of grains of this character than will other methods of milling. If the grits are screened by the-usual methods, using a prolonged time anda fairly heavy impact or jolt during screening, and if the material is then rescreened for a short time with ,only a light vibratory motion, most of the elongated particles particles'will pass through the meshes;

will remain on the screen, Whereas the equiaxed Other methods suchas air Winnowing, separation of graded grits on the basis of their difference in weight, and similar processes of separation are also available for obtaining particles having different dimensions with respect to, their three These methods are well known in the art and do not require detailed description. 7 If silicon carbide is used as the abrasive, it is possible to obtain grain in which a substantial proportion of the particles are more or less perfectly formed fiat plates or sharp elongated crystals, providing the crystal aggregates. as they come from the furnace are broken up and screened without resorting to long continued grinding or mulling to obtain the desired grit sizes;

The various'types of apparatus shown in the accompanying drawing are suitable for effecting an orientation of the particles in the mix prior to or during consolidation. The grains can be wheel.

mixed or coated with a bonding agent before subjecting them to the orienting process, or the In order to obtain a wheel in which the pa ticles are oriented radially, amold of the general type shown in Figure 1 can be used. Referring to Figure 1, the mold comprises a metal barrel 2 and a bottom plate 3 supported by the base [0. The bottom plate 3 is preferably composed of an insulating material of high dielectric strength, as for example, hard rubber. Centrally positioned within the mold is a metal electrode 4, which is surrounded with asheath 5. The char-' acter of the sheath 5 appears to be of great importance in the operation of our apparatus and preferably is made of some" poorly conducting material. The materials should be so poorly conducting that not enough current can flow along or through it to cause arcing between the electrode surfaces 4 and 2 and yet it should be sufliciently conducting to permit the passage of sumcient leakage or induced electricity to impart a charge on the granular particles as they are deposited in the mold.

When We speak of a poorly conducting material, we use this expression to distinguish between such materials as metals, carbon, or the like, which are relatively good conductors of electricity on the one hand, and highly insulating materials such as glass, hard rubber, varnished cambric, or the like, which are good insulators at the voltages we prefer to apply to our apparatus that the leakage current which flows through them is insufficient to charge the granular material to the degree necessary to cause it to be radially oriented upon deposition in the mold, on the other hand.

For this purpose we have found ordinary dry, knot-free wood to be among those materials satisfactory. While dry wood is so poorly conducting that it will not carry enough current to allow spark over or arcing between the bottom and top electrodes, it is conductive enough to allow sufiicient leakage at the operating voltages to supply the small amount of electrostatic charges required for the orientation of the abrasive particles. Other woods and'other materials that are ordinarily classed as insulators but which are in fact poor conductors of high tension electric current may also be employed. Such 'materials include fibrous material bonded with a resinous material such as phenolic condensation product resin. Such fibrous material should not have insulating laminations through it but If it should be desired to subject the wheel tov higher consolidating pressures than'the insulation will withstand, the bottom plate 3 and the poorly conducting electrode can be covered with paper, and can be removed and replaced with similar steel parts after the orientation of the grain and the preliminary consolidation of the of the mold. A source of potential of, for example, 40,000 volts has-been foundsatisfactory The outer shell 2 contacts with the terf for most mixes, although lower or higher voltages over a considerable range can be used as the occasion demands.

A uniform distribution of the granular material and bond may be obtained by screening or sifting the mix into the mold. This is preferably done by sifting the mix through a vibrating or a gyratory riddle or screen suspended above the mold as illustrated in Figure 1. The riddle or screen i is preferably provided with a lower screen it and one or more upper screens 9. The riddle or screen is vibrated as uniformly as possible and at the correct speed to cover the surface of the riddle with mix.

As the mix is fed through the screens of the riddle the granular particles are individually separated and fall into the mold in the best condition for orientation. The granular particles become electrically charged by the electrostatic field set up between the shell 2 and the electrode 5, as they are deposited into the mold. The granular particles may receive an electrical charge by electrostatic induction, by corona discharge from the highly charged electrode sur face 5 or in some few instances by direct contact with said electrode. It is believed that the electrical charge produced on the elongated granules congregates toward the opposite ends where the curvature is greatest, thus forming electrical doublets. Forces produced by the electrical field then turn the charged elongated granules so that their major axes are parallel to the lines of force of the electrostatic field and causes the longitudinally oriented granules to be deposited in the mold in a radially oriented position.

Vibrating or tapping the mold during the step of filling the mold with granular mix is very desirable since it produces a more even distribution of the granular particles and causes a preliminary settling or consolidation of the mix.

If a ceramic or vitrifiable bond is used, it is possible to orient the grains while the mix is in a dry condition. The dry ingredients of the bond can be finely powdered and thoroughly mixed, and then mixed with the abrasive grain. If care is taken to prevent segregation of the bond, the dry mix can be scattered into the mold while the potential is applied, and we have found that when this procedure is followed, the grains orient themselves so that their greatest axis is in a radial position with respect to the wheel. Moisture can be applied to the mix after orientation, although moistening should be effected very gradually owing to the tendency of the added water to wash the bond from the surface of the grains and to cause segregation of the bond in the lower portions of the wheel. If moisture is applied in a very finely atomized spray or by condensation from a moisture or steam laden atmosphere, the mixture can be rendered sufficiently damp for final consolidation. With most organic bonds the application of moisture is, of course, unnecessary.

In view of the difiiculties encountered from segregation of the bond in a dry mix, it is often desirable to coat the abrasive particles with a fairly adherent layer of bond prior to orientation. Various methods can be employed for producing such a coating. In the manufacture of a resin bonded wheel, the grains can be Wet with a plasticizer or resin solvent, as for example, furfural, and the dry powdered resin thoroughly and rapidly mixed with the grain. The resulting mix is of a dry consistency and can be seattered into the mold and'the particles oriented by means of an electrostatic field.

When it is desired to coat the grain with an inorganic bond, the grain can be coated with a material to which the finely divided solid bond particles will adhere. The grain, for example, can be heated and intimately mixed with a small quantity of stearic acid, and subsequently mixed with the powdered bonding material. The bond can also be mixed with a small quantity of creamy emulsion such as an emulsion of stearic acid in water, in order to render the bond adherent, whereupon the resulting mixture can be thoroughly mixed with the grain and dried so as to produce an adherent coating of bond over the particles. t is also possible to form a damp mix containing an agglutinant and to subsequently dry and distribute the particles in the mold in a dry condition.

Figure 2 illustrates diagrammatically a meth- 06. for obtaining an orientation of the particles within a solid article wherein the major axes of the particles are positioned in the same direction, although the fiat surfaces of the particles may not necessarily lie in parallel planes. A mold i2 is positioned between the condenser plates 13 and M which are connected to a source of high potential to produce a strong electrostatic field and the mix can be scattered into the mold under the action of an electrostatic field and subsequently consolidated.

In scattering the mix into a mold under the action of an electrostatic field, the degree of consolidation obtained thereby may not be suificient for the purpose desired. In order to secure a greater degree of consolidation without destroying the orientation produced by the electrostatic field, the molds shown in Figures 1 and 2 can be vibrated or jolted during the scattering of the mix into the mold, or after the filling of the mold is complete. The jolting action causes a settling of the particles within the mold, and if jolting is carried out while the particles are under the action of an orienting field, the orientation of the particles can be combined with a more dense consolidation than can be obtained by merely scattering the mix into the mold. Jolting itself produces an orientation of the particles in parallel planes as will be further explained.

In order to make a wheel in which the bond is introduced in a liquid condition, the loose grains can be oriented and consolidated by any of the methods previously given. The bond is then dissolved in a solvent, which is preferably volatile, and is poured into the pores of the mix. As an example of a specific method of producing such a wheel, an organic bond such as shellac can be dissolved in alcohol or other suitable solvent and introduced into the pores of the article. A solution of 50 grams of shellac in 150 cc. of alcohol is satisfactory. After evaporation of the alcohol, the bond can be cured in the manner well known in this particular art.

In addition to radial orientation, we may obtain a parallel orientation of the particles With-q in an abrasive wheel by means of a jolting operation. In order to eifect such orientation, a plastic mix rather than the usual dry press mix should be used. As an example of such a mix, any of the usual puddle mixes such as are used in making puddled abrasive wheels can be employed, except that slightly less water should be used than is the case with puddling. The regular puddle mix containing the usual amount of A moresatisfactory method of forming a plastic mix comprises introducing the powdered bond into a carrier such as an emulsion of stearic acid,

oil, or a similar decomposable substance in water. The consistency of these emulsions can be varied over a wide range, and the mixture of bond and emulsion can be made so as to have about the same consistency as cold cream. The amount of emulsiorrused can be proportioned so as to fill all of the pore space in the consolidated mix. After jolting, the wheel is dried, the volatile matter carefully driven off, and the article cured in the usual manner.

A third method of forming a plastic mix for the orientation of abrasive particles by jolting comprises using asa bond an artificial dispersion of rubber having a thick creamy consistency.

The manufacture of such a dispersion and its use as an abrasive bond is described in an application of R. C. Benner, Patent No. 1,969,497, issued August 7, 1934.

When the abrasive mix is jolted and a bond is used which is sufiiciently plastic, the individual particles orient themselves so that their greatest dimension is at right angles to the direction of the jolting. The method is especially applicable for the orientation of flat plates. With grain of this character, a wheel or other article having very sharp projecting edges can be produced.

We claim: 7 V 1. An apparatus. for forming bonded granular articles comprising electrodes arranged to form a mold for the said articles, means for applying a high electrical potential difference between the said electrodes and maintaining an intense electrostatic field therebetween, means for depositing elongated granular materials and a bond therefor into said mold and between said electrodes, said apparatus being adapted to electrostatically orient the elongated granular particles so that their longest dimensions lie in a predetermined direction.

2. An apparatus for forming bonded granular articles comprising electrodes arranged to form a mold for said article, one 'ofsaid electrodes having a facing of poorly conducting material to permit a leakage current to fiow .therethrough but of sufiicient resistance to' prevent arcing or spark over between said electrodes, means for applying a high electrical potential difference between the said electrodes, means for depositing elongated granular materials and a bond therefor into said mold and between said electrodes, said apparatus being adapted to electrostatically orient the elongated granular particles so that their longest dimensions lie in a predetermined direction.

3. An apparatus for forming bonded granular articles comprising electrodes arranged to form a mold for said articles, one of said electrodes composed of a good electrical conductor and spaced apart from an electrode of sufiicient conductivity to permit a leakage current to flow therethrough but of suflicient resistance to prevent spark-over or arcing between said electrodes, means for depositing elongated granular partlcles and a bond therefor into said mold and between said electrodes, said apparatus being adapted to electrostatically orient the elongated granular particles so that their longest dimensions lie in a predetermined direction.

4. An apparatus for forming bonded granular articles comprising electrodes and a base member arranged to form a mold for said articles, one of said electrodes composed of a good electrical conductor and spaced apart from an electrode of sufficient conductivity to permit a leakage current to flow therethrough but of suflicient resistance to prevent arcing between said electrodes, said base member being adapted to support said electrodes in operative position and composed of a good electrically insulating material, means for depositing elongated granular particles and a bond therefor into said mold and between said electrodes, said apparatus being adapted to electrostatically orient the elongated stantial thickness is produced, said apparatus,

being adapted to electrostatically orient the elongated abrasive particles so that their longest dimensions lie in a predetermined direction.

ROMIE L. MELTON. HENRY P. KIRCHNER. RAYMOND C. BENNER. 

